Problem:

UNIX_TIMESTAMP() called for a expression of the TIME data type
returned NULL.

Inside Type_handler_timestamp_common::Item_val_native_with_conversion
the call for item->get_date() did not convert TIME to DATETIME
automatically (because it does not have to, by design).
As a result, Type_handler_timestamp_common::TIME_to_native() received
a MYSQL_TIME value with zero date 0000-00-00 and therefore returned "true"
(indicating SQL NULL value).

Fix:

Removing the call for item->get_date().
Instantiating Datetime(item) instead.
This forces automatic TIME to DATETIME conversion
(unless @@old_mode is zero_date_time_cast).